1. Field of the Invention
The present invention relates to a light interception structure for an AF sensor of an automatic focus detection surveying instrument.
2. Description of the Related Art
In a, conventional auto-focus surveying instrument having a collimating telescope, such as a total station, an optical system is composed of a collimating optical system including an objective lens, a focusing lens, a Porro prism, a focusing plate and an eyepiece in this order from the object side, and a beam splitting optical system including a beam splitter provided in the vicinity of the Porro prism to split a light path of a focus detection optical system from a light path of the collimating optical system. In the focusing system, the focus state of a plane which is optically equivalent to a focal plane of the collimating optical system located within the focus detection optical system is detected by a phase difference detection type auto-focus sensor having a pair of CCD sensors to calculate the amount of defocus (defocus amount), so that a focusing lens is moved to a focal position in accordance with the defocus amount to carry out the automatic focusing (AF) operation.
However, in the conventional structure, since the beam splitting system and the Porro prism are located on the optical axis of the collimating optical system, if external light enters the collimating optical system through the eyepiece, the external light reaches the AF sensor due to internal reflections, so that the external light overlaps the light emitted from the objective lens, thus resulting in an incorrect focusing operation.